The U.S. environmental movement has devoted immense effort and political capital to blocking Keystone XL, the pipeline that would bring bitumen from Canadian oil sands to Gulf Coast refineries. James Hansen, the most visible climate scientist to turn anti-carbon campaigner, declared it “game over” for climate change if the pipeline is approved. Yet Keystone is a far from obvious focus for the limited campaign resources available to protect our climate.

Oil-sands fuels emit about 10 to 20 percent more carbon than conventional oil over their life cycle. Suppose Keystone is blocked, or suppose that technical fixes (see “Alberta’s Oil Sands Heat Up” ) eliminated that extra carbon. Neither outcome would make a big dent in emissions. The root of the climate threat posed by fuels is the carbon emitted when they are burned for transportation. Emissions from vehicles account for more than three-quarters of the life-cycle emissions of petroleum fuels. To solve the problem, we must decouple transportation from petroleum.

Moreover, the oil market is adaptable, so if Keystone is blocked, Canadian bitumen will still find its way to world markets, perhaps by a Pacific route, leaving emissions essentially unchanged.

For all that, there is a sound strategic reason for environmental groups to try to kill oil-sands development. Without constraints, the production of such unconventional hydrocarbon fuels will increase quickly in the coming decades as conventional supplies shrink. This will depress oil prices, meaning that efficiency improvements and alternatives such as electric vehicles will continue to have trouble competing. The oil-sands boom is the biggest unconventional-oil project yet, and Keystone is a centerpiece of that boom.

That’s not what you’ll hear campaigners say about Keystone, though. Tactics and strategy get confused in the heat of battle, and many of the public arguments against the pipeline are spurious. The threat of pipe leakage, to cite a recently hyped example, is minuscule compared with the environmental risks posed by other new developments in energy, from shale gas to large-scale corn ethanol.

I hope the Keystone permit is denied, because the strategic case for blocking huge capital investments in unconventional fuels is compelling. Yet I worry that the environmental movement has overinvested in a battle it seems unlikely to win—one whose tactics distract attention from the hard choices needed to decarbonize transportation and accelerate energy innovation.

David Keith is a professor of public policy and applied physics at Harvard University and president of Carbon Engineering, a startup developing carbon- capture technology.